More powerful and brighter LED light sources are in high demand for challenging applications such as projection systems (rear and front projection televisions). Prior projection systems typically utilized short arc lamps such as high-pressure mercury, metal halide, and xenon. These lamps are capable of very high luminous emittance and are therefore suitable sources for the étendue limited projection systems and are used almost exclusively in today's commercial front and rear projection systems. However, there are problems associated with these lamp technologies including poor luminous efficacy, thereby requiring high power and associated means of cooling, short lifetime, poor color gamut (number of colors that can be reproduced given the spectrum of the lamps), high voltage operation (typically kilovolts required to turn them on), high cost, and use of mercury, which is becoming an environmental hazard and is in the process of undergoing regulations in numerous countries throughout the world.
Only recently has there been recognition that LEDs could be used to replace more traditional light sources in video projection systems. In particular, LEDs provide much improved lifetime, color gamut, lower cost of ownership, lower power consumption (enabling some battery operated portable devices), decreased cooling requirements, and freedom form mercury relative to conventional arc lamps. To date, there have been several companies that have demonstrated future products based on LED sources in projection systems. The luminous emittance of these systems, however, has not been sufficient. The LED projection system embodiments described herein provide much improved luminous emittance and lifetime relative to existing LED based systems.
U.S. Pat. No. 7,153,015 by Brukilacchio describes the use of LED die or LED die array substantially filling the input aperture of a dielectric filled non-imaging concentrator with a compound parabolic profile and a high performance thermal interface. This type of configuration and derivatives thereof can be effectively used in projection systems. Brukilacchio describes a concentrator with a substantially circular cross section. While this device is described as having an efficiency of 96% with respect to the light entering the input aperture of the concentrator leaving the exit aperture within the desired solid angle while substantially preserving the étendue, the circular cross section concentrator is not a good match to the aperture of typical rectangular format video modulation devices. Thus, for example, a XGA format (1024 by 768 pixels) aperture inscribed in a circle for which the vertices of the rectangle lie on the diameter of the circle would waste approximately 40% of the light emitted from an associated LED source with a concentrator of circular cross section.
Accordingly, it is a primary object of the present invention to provide projection systems that efficiently utilize the output from one or more LED sources.
It is another object of the present invention to provide an LED projection illumination system that efficiently matches the étendue of an LED source to that of a digital modulation device (DMD) for forming images.
It is another object of the present invention to provide devices for efficiently coupling the outputs from a plurality of LED sources and directing them along a common path.
It is another object of the present invention to provide specially configured non-imaging optics for forming circular far fields from LEDs having non-circular geometries.
Other objects of the invention will be obvious and will appear hereinafter when the following detailed description is read in conjunction with the drawings.